GB2182676A

GB2182676A - Master alloy for titanium-based alloys

Info

Publication number: GB2182676A
Application number: GB08626410A
Authority: GB
Inventors: Hermann Andorfer; Reinhard Hahn
Original assignee: ELEKTROMETALLURGIE GmbH; GfE Gesellschaft fuer Elektrometallurgie mbH
Current assignee: ELEKTROMETALLURGIE GmbH; GfE Gesellschaft fuer Elektrometallurgie mbH
Priority date: 1985-11-06
Filing date: 1986-11-05
Publication date: 1987-05-20
Also published as: GB2182676B; US4684506A; DE3635194A1; JPS63100150A; GB8626410D0; JPH0465137B2; DE3635194C2

Description

1 GB 2 182 676 A 1

SPECIFICATION

Master Alloy for the Production of Titanium-based Alloys and Method for Producing the Master Alloy The present invention relates to a master a] loy for 70 the production of a titanium-based alloy, the master alloy having a molybdenum content in excess of 20% by weight, a vanadium content in excess of 10% by weight and an aluminium content in excess of 40% by weight. It also relates to a process for making this master alloy and to a process for making the titanium-based alloy utilizing the master alloy.

It is known to provide a master alloy (see United States Patent 3 387 971) which has a molybdenum content of 20 to 25% by weight, a vanadium content of 20 to 25% by weight, no titanium, and the balance of aluminum.

This master alloy is formed in a single stage and its melting point is determined, by the fact thatthe content of molybdenum plus vanadium plus aluminum is always at least 99% as a result of the limited content of carbon, oxygen, nitrogen, and hydrogen, to be less than 14000C. With a high molybdenum content of the master alloy, however, problems arise in that molybdenum is only soluble with considerable difficulty in the titanium-based alloy.

The production of titanium-based alloys from master alloys deserves some comment.

Titanium-based alloys containing the elements aluminum, molybdenum and vanadium in different compositions and ratios are commercially significant because of their utility in the fabrication of aircraft and vehicles for space travel. Thus, it is especially important in the fabrication of titaniumbased alloys that the alloying elements in the base metal be distributed with an optimum homogeneity so that properties of the metal bodies are substantially isotropic.

Especially metals having high melting points, or refractory metals such as molybdenum with a melting point of 26100C, are difficuitto dissolve homogeneously in the lower melting titanium whose melting point is only 16680C.

Experience has shown that existing aluminum master alloys containing molybdenum have not fully solved this problem. Such aluminum master alloys include All,Mo, Al,Mo, Al,Mo, AlpMo and A[o3. Even with these alloys it is difficult to bring about complete and homogeneous dissolution of molybdenum, even in the form of the master alloy, in the titanium.

Unclissolved molybdenum compounds and unmelted molybdenum particles, when distributed in the titanium-based structure, create problems in fabrication and as to the strength of the pieces made from the alloy, because at the inclusion sites of the undissolved alloy or the particles crackformation can occur. The ageing properties of the product are poor, the fatigue resistance is low and, in general, practically all of the strength properties are adversely affected.

It is possible to approximate a satisfactory degree of homogeneity in titanium-based alloys by 130 providing the alloying elements in appropriate master alloys and then mixing with with titanium sponge, and pressing the products at sufficient pressures to shaped articles. These shaped articles are then converted by welding in special processes to melting electrodes, which are transformed by electric arc furnace melting to ingots and, utilizing various ingot remelting techniques, the homogeneity of the resulting titanium-based alloys can be increased. These methods are extremely complex and frequently onerous.

It is the principal object of the invention to provide an improved master alloy which will avoid the drawbacks mentioned above.

Another object of this invention is to provide a master alloy which has a relatively low melting temperature and yet a high molybdenum content, so that it can be used in the fabrication of especially homogeneous titaniumbased alloys with improved properties and without the very complex techniques hitherto required to ensure homogeneity.

Still another object of the invention is to provide a master alloy of high molybdenum content with especially high solubility in titanium in the formation of a titanium-based alloy.

A further object of the invention is to provide an improved method of making a low melting master alloy capable of introducing relatively large amounts of molybdenum into titanium-based alloys.

According to the invention, a master alloy for the production of titaniumbased alloys is formed with a molybdenum content of 25 to 30% by weight, a vanadium content of 15 to 18% by weight and the molybdenum content is at least 1.4 times the vanadium content, the alloy additionally containing up to 7% by weight titanium, the balance aluminum, the master alloy having a melting point below 15000C. Most advantageously, the Mo content will be above 25% by weight and normally at least 27% by weight.

While it is possible for the alloy to have no titanium, preferably the master alloy of the invention has more than 1 % by weight titanium and, in the most preferred state, has about 7% by weight titanium although deviations by about 1 % by weightfrom this latter value are tolerable.

The master alloy itself has been found to be extremely homogeneous.

However, possibly the most surprising characteristic of the invention is that, by the aforestated relationship of the molybdenum content to the vanadium content, exceptionally high molybdenum contents can be provided in the master alloy, which has an exceptionally good solubility in titanium with substantially complete dissolution of the molybdenum in the titaniumbased alloy. This is indeed surprising where the molybdenum content exceeds 25% by weight.

The master alloy of the invention has other advantages as well. For example, it can be comminuted easily and with low energy consumption.

Mention may be made of the fact that master alloys for the production of titanium-based alloys 2 GB 2 182 676 A 2 which contain small amounts of titanium have been described in the art (DE-OS 28 21 406) but the alloys of this publication are not equivalent to those of the present case and indeed appear to be relevant only to alloys which are to have significant 70 zirconium contents.

The master alloy of the invention can be made in various ways. What is believed to be the best mode embodiment of the invention is aluminothermally producing molybdenum/aluminum alloy and vanadium/aluminum alloy, which thus have a high degree of purity, melting the molybdenum/ aluminum alloy,the vanadium/aluminum alloy, aluminum metal and titanium metal in a vacuum induction furnace, and casting the resulting melt.

Preferably the molybdenum/aluminum alloy consists substantially of 75% by weight of molybdenum and substantially 25% by weight of aluminum, the vanadium/aluminum alloy consists substantially of 80% by weight of vanadium and substantially 20% by weight of aluminum, the aluminum metal has a purity of 99.8% by weight of aluminum, and the titanium metal has a purity of 99.7% by weight of titanium. The vacuum induction furnace is preferably operated so that melting is effected with inductive movement of the melt and, after vacuum degassing, melting is continued under a protective gas, e.g., argon, and preferably with inductive bath agitation or stirring until all detrimental aluminum oxide inclusions are removed aluminothermally and a highly homogeneous product is obtained.

The master alloy can then be cast at temperature of at most 15100C under argon and then cooled under helium at a pressure of at most 200 torr.

The titanium-based alloy can be made by vacuum melting andlor in an electric arc furnace, the solidified master alloy having titanium in the desired proportions for the titanium-based alloy of interest.

A preferred method of producing the four component master alloy of the invention utilizes a two-stage process which has been found to ensure an especially dense and inclusion-free master alloy of high homogeneity. Melting in the second stage is 110 effected in a vacuum induction furnace which reduces the impurity content of the product to especially low levels, for example a maximum of 0.008% nitrogen and substantially 0.02 to 0.04% oxygen. In the first stage of the process, molybdenum/aluminum and vanadium/aluminum alloys are formed by aluminorthermal reduction in a burn-off even by furnace by, for example, intimately mixing relatively high purity molybdenum (V1) oxide (M003) with a purity of at least 99.9% M003 with 120 high purity aluminum and then igniting the reaction mixture.

The aluminothermal reaction ensures an effective separation of the metal from the slag, and the addition of a fluxto reduce the viscosity of the slag is not necessary. This is highly important because the elimination of the need for a slag also avoids an opportunity to introduce additional contaminants.

Depending upon the stoichiometry of the mixture and reaction the alloy can contain 72to 75% 130 molybdenum and 28 to 25% aluminum by weight. The aluminum is of course added in excess to allow burn-off by the oxygen of the M003 or the V20, In the same way high purity V205 is reacted with aluminum to produce the vanadium/aluminum alloy aluminorthermally containing 80 to 82% by weight vanadium, 20 to 18% by weight aluminum.

The second stage melting is carried out as described in a vacuum melting furnace with the starting material MoAl 75:25, VAI 80:20,99.9% purity aluminum and titanium metal of 99.7% purity which are introduced through a vacuum gate into the ceramic crucible and there heated inductively with inductive stirring. After degasification, an argon-protective atmosphere is applied and the stirring of the melt continued in a refining operation to remove even minimal A1203 inclusions. The bath movement ensures optimum homogeneity. The melting process is controlled precisely with monitoring of the melting temperature and the melt is then cast at no more than 1510'C in steel ingot moulds under argon and cooling is effected under helium at no more than 200 torr.

By way of example, two charges are formed (all percent by weight).

Charge 1 Into the vacuum induction furnace are introduced 4.728 kg MoAl 1.852 kg VAI 0.702 kg Ti-scrap 2.718 kg A[-granules 73.6% Mo 80.5% V 99.7% Ti 99.7% Ti The mass is melted, degassed and maintained in a liquid stage. The casting is effected at 1510'C under argon and the ingots are cooled for three hours under helium at a pressure of 200 Torr.

The product is:

9.51 kg of AI-Mo-V-Ti 43:35:15:7 (percent by weight) with 41.5 % AI 35.8 %Mo 15.1 % V 6.9 % Ti 0.20 % Fe 0.08 % si 0.022%0, 0.007% N, 0.016% C 0.001 % E 0.015% Cr 0.002% Cu 0.002% Mg 0.003% Mn 0.009% Ni 0. 008% p 0.001%S 0.001 % Pb 0.03% W 0.002% Y Solid us-Temperatu re 1420 10'C Liquidus-Temperature 1460 150C 3 GB 2 182 676 A 3 Charge 2 Into a vacuum induction furnace, the following are introduced:

3.588 kg MoAl 2.210 kg VAI 0.702 kg T-scrap 3.514 kg M-granules 75.0% Mo 81.0% v 99.7% Ti 99.7% A] Melting was effected as with Charge 1. However 60 the casting temperature was 142WC and the product obtained was:

9.85 kg N-Mo-V-Ti 48:27:18:7 (percent by weight) with 48.3 % AI 26.1 % Mo 17.9 % V 7.1 % Ti 0.22 % Fe 0.075% Si 0.028%02 0.008% N2 0.01 % c 0.001% B 0.013% Cr 0.001 % Cu 0.002% Mg 0.004% Mn 0.005% Ni 0. 007% P 0.001%S 0.001 % Pb 0.01 % W 0.001 % Y Solidus-Temperature 1330 20'C Liquidus-Temperature 1365 20'C.

The master alloys of Charges 1 and 2 were readily comminuted in a hammermill and melted in a vacuum furnace or an electric arc furnace with 90 titanium to form titinium-based alloys having a high molybdenum content. The titanium alloys were found to be highly effective in aircraft and space vehicles.

Typical of the alloys which were produced were alloys containing 6% by weight and more molybdenum, vanadium in an amount determined by the master alloy ratio to the molybdenum, aluminum in an amount determined by the master alloy ratio, and titanium.

Claims

1. A master alloy for the production of titaniumbased alloys which consists essentially of 20 to 30% by weight molybdenum, 15 to 18% by weight vanadium, up to about 7% by weight titanium, the balance aluminum, and wherein the molybdenum content is by weight at least 1.4 times the vanadium content and the master alloy has a melting point below 150WC.

2. A method of making a master alloyforthe production of titanium-based alloy which consists essentially of 25 to 30% by weight molybdenum, 15 to 28% by weight vanadium, up to about 7% by weight titanium, the balance aluminum, and wherein the molybdenum content is by weight at least 1.4 times the vanadium content and the master alloy has a melting point below 1500'C, which comprises the steps of: aluminothermally producing molybdenum/aluminum alloy and vanadium/aluminum alloy; melting the molybdenum/aluminum alloy, the vanadium/ aluminum alloy, aluminum metal and titanium metal in a vacuum inducation furnace; and casting the resulting melt.

3. A method as defined in Claim 2 wherein the molybdenum/aluminum alloy consists substantially of 75% by weight molybdenum and substantially of 25% by weight aluminum, the vanadium/aluminum alloy consists substantially of 80% by weight vanadium and substantially of 20% by weight aluminum, the aluminum metal has a purity of 99.8% by weight of aluminum, and the titanium metal has a purity of at least 99.7% by weight of titanium.

4. A method as defined in Claim 2 or Claim 3 wherein the melting in the vacuum induction furnace is effected with inductive movement of the melt in the furnace, and after vacuum degassing, melting is continued under a protective gas until all detrimental aluminum oxide inclusions are removed aluminothermally and a homogeneous product is obtained.

5. A method as defined in Claim 4 wherein the homogeneous molten product is cast at a temperature of at most 151 WC under argon and then cooled under helium at a pressure of at most 200 to rr.

6. Master alloys produced substantially as hereinbefore described with reference to the 100 examples.

7. Methods of making master alloys substantially as hereinbefore described.

Printed for Her Majesty's Stationery Office by Courier Press, Leamington Spa, 511987. Demand No. 8991685. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.